Recently, demand on the water treatment technology is increasing globally due to the insufficiency of water resources and worsening of water quality. To meet this demand, improvement of existing water treatment facilities or introduction of new filtration devices and filtration processes is being attempted. However, the improvement of existing water treatment facilities or introduction of new advanced treatment facilities has many problems in site acquisition, cost, etc. Therefore, advanced water treatment processes using membrane filtrations, which are capable of providing stable water quality and can be operated and maintained easily, are being presented.
The membrane filtration water treatment method separates pollutants from raw water using a separation membrane having selective permeability. This is advantageous in that suspended materials of a certain size or larger included in the raw water can be removed surely. However, this process shows decrease in membrane filtration performance due to a layer formed by pollutants, solids, etc. on the surface of the separation membrane. In this case, the membrane filtration performance can be recovered by washing the separation membrane physically through water backwashing, air backwashing, etc. or chemically through decomposition or dissolution using chemicals.
Typically, a separation membrane used in the filtration device of a water treatment facility consists of hundreds or thousands of hollow fibers. The hollow fiber type separation membrane is a membrane which is capable of removing suspended materials, bacteria, etc. contained in raw water. But, it is important to occasionally check any damage to the separation membrane in order to ensure good water quality. If damage occurs on part of the separation membrane for some reason, it is highly likely that pollutants may leak out of the separation membrane. Accordingly, it is important to detect and deal with the damage to the separation membrane at an early stage of a membrane filtration process.
Technologies for detecting damage to the separation membrane in a membrane filtration process are disclosed.
For example, methods of injecting air to a primary side, which is in gas state whereas a secondary side is in liquid state, to detect damage to the separation membrane are disclosed in Journal AWWA, Desalination and Journal of Membrane Science. And, Japanese Patent Publication Nos. 2000-342936, 2001-269551 and 2007-245060 disclose methods for detecting damage to the separation membrane by injecting air to a primary side or a secondary side of a membrane module consisting of hollow fibers, with the primary side and the secondary side maintained in gas state and liquid state, respectively.
According to the above-described literature and patent documents, the damage to the separation membrane is detected in the state where the primary side and the secondary side of the membrane module are maintained at gas and liquid states or liquid and gas states, respectively. In such situations, compressed air moves from the gas state side to the liquid state side through the damaged part of the separation membrane due to differential pressure. However, the change in air pressure as the air moves from the gas side to the liquid side is not large due to resistance caused by density, partial air pressure, etc. Even if the damage to the separation membrane can be detected by the above-described methods, it is very difficult to find the damaged part of tens of micrometers from the module consisting of hundreds to thousands of hollow fibers. For this reason, the damaged module is replaced entirely in the actual process.
Meanwhile, a method of injecting liquid silicone and then curing the same or inserting a small nail or pin into the damaged hollow fiber membrane is used to repair the damaged hollow fiber membrane in a membrane filtration process.
However, the method of injecting and curing liquid silicone has problems in that a cassette of the relevant module must be placed on a worktable during the repair operation due to the fluidity of the silicone and in that the repair is carried out for a long time due to a long curing time, thereby causing increased time and labor and decreased workability. And, the method of inserting a nail or pin into the hollow fiber membrane has a problem in that, when the hollow fiber membrane is a reinforcing membrane, a small gap can be created between the membrane and the nail or pin, causing contamination of filtered water. In particular, for a pressurized module having a housing, the hollow fiber membrane cannot be blocked completely due to a narrow space created the fine edge or blade and the repaired membrane.